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It depends on the game. In BW, you have to have beaten the Elite Four, N and Ghetsis. You will want to make sure you go through the Giant Chasm so that you reach a second cave. Kyurem should be there at Lv70.
In B2W2, you must first capture/beat Zekrom (Black 2) or Reshiram (White 2) at the top of Dragonspiral Tower (after beating N at his castle, through the exit where the Zoroark was. Head over to the part where Ghetsis was controlling Kyurem and it should be there at Lv70. Dusk Balls and Quick Balls help, or you could play it very safe with the Master Ball.
BONUS: Once you have captured/beat Kyurem, you can find a new Key Item: the DNA Splicers, which is used to change Kyurem's Forme into Black Kyurem or White Kyurem